Rotary steerable systems (RSS) currently used in drilling oil and gas wells into subsurface formations commonly use tools that operate above the drill bit as completely independent tools controlled from the surface. These tools are used to steer the drill string in a desired direction away from a vertical or other desired wellbore orientation, such as by means of steering pads or reaction members that exert lateral forces against the wellbore wall to deflect the drill bit relative to wellbore centerline. Most of these conventional systems are complex and expensive, and have limited run times due to battery and electronic limitations. They also require the entire tool to be transported from the well site to a repair and maintenance facility when parts of the tool break down. Most currently-used designs require large pressure drops across the tool for the tools to work well. Currently there is no easily separable interface between RSS control systems and formation-interfacing reaction members that would allow directional control directly at the bit.
There are two main categories of rotary steerable drilling systems used for directional drilling. In “point-the-bit” drilling systems, the orientation of the drill bit is varied relative to the centerline of the drill string to achieve a desired wellbore deviation. In “push-the-bit” systems, a lateral or side force is applied to the drill string (typically at a point several feet above the drill bit), thereby deflecting the bit away from the local axis of the wellbore to achieve a desired deviation.
Rotary steerable systems (RSS) currently used for directional drilling focus on tools that sit above the drill bit and either push the bit with a constant force several feet above the bit, or point the bit in order to steer the bit in the desired direction. Push-the-bit systems are simpler and more robust, but have limitations due to the applied side force being several feet from the bit and thus requiring the application of comparatively large forces to deflect the bit. As a matter of basic physics, the side force necessary to induce a given bit deflection (and, therefore, a given change in bit direction) will increase as the distance between the side force and the bit increases.
Examples of prior art RSS systems may be found in U.S. Pat. No. 4,690,229 (Raney); U.S. Pat. No. 5,265,682 (Russell et al.); U.S. Pat. No. 5,513,713 (Groves); U.S. Pat. No. 5,520,255 (Barr et al.); U.S. Pat. No. 5,553,678 (Ban et al.); U.S. Pat. No. 5,582,260 (Murer et al.); U.S. Pat. No. 5,706,905 (Barr); U.S. Pat. No. 5,778,992 (Fuller); U.S. Pat. No. 5,803,185 (Barr et al.); U.S. Pat. No. 5,971,085 (Colebrook); U.S. Pat. No. 6,279,670 (Eddison et al.); U.S. Pat. No. 6,439,318 (Eddison et al.); U.S. Pat. No. 7,413,034 (Kirkhope et al.); U.S. Pat. No. 7,287,605 (Van Steenwyk et al.); U.S. Pat. No. 7,306,060 (Krueger et al.); U.S. Pat. No. 7,810,585 (Downton); and U.S. Pat. No. 7,931,098 (Aronstam et al.), and in Int'l Application No. PCT/US2008/068100 (Downton), published as Int'l Publication No. WO 2009/002996 A1.
Currently-used RSS designs typically require large pressure drops across the bit, thus limiting hydraulic capabilities in a given well due to increased pumping horsepower requirements for circulating drilling fluid through the apparatus. Point-the-bit systems may offer performance advantages over push-the-bit systems, but they require complex and expensive drill bit designs; moreover, they can be prone to bit stability problems in the wellbore, making them less consistent and harder to control, especially when drilling through soft formations.
A push-the-bit system typically requires the use of a filter sub run above the tool to keep debris out of critical areas of the apparatus. Should large debris (e.g., rocks) or large quantities of lost circulation material (e.g., drilling fluid) be allowed to enter the valve arrangements in current push-the-bit tool designs, valve failure is typically the result. However, filter subs are also prone to problems; should lost circulation material or rocks enter and plug up a filter sub, it may be necessary to remove (or “trip”) the drill string and bit from the wellbore in order to clean out the filter.
For the foregoing reasons, there is a need for rotary steerable push-the-bit drilling systems and apparatus that can deflect the drill bit to a desired extent applying lower side forces to the drill string than in conventional push-the-bit systems, while producing less pressure drop across the tool than occurs using known systems. There is also a need for rotary steerable push-the-bit drilling systems and apparatus that can operate reliably without needing to be used in conjunction with filter subs.
Push-the-bit RSS designs currently in use typically incorporate an integral RSS control system or apparatus for controlling the operation of the RSS tool. It is therefore necessary to disconnect the entire RSS apparatus from the drill string and replace it with a new one whenever it is desired to change bit sizes. This results in increased costs and lost time associated with bit changes. Accordingly, there is also a need for push-the-bit RSS designs in which the RSS control apparatus is easily separable from the steering mechanism and can be used with multiple drill bit sizes.
There is a further need for push-the-bit RSS systems and apparatus that can be selectively operated in either a first mode for directional drilling, or a second mode in which the steering mechanism is turned off for purposes of straight, non-deviated drilling. Such operational mode selectability will increase service life of the apparatus as well as the time between tool change-outs in the field. In addition, there is a need for such systems and apparatus that use a field-serviceable modular design, allowing the control system and components of the pushing system to be changed out in the field, thereby providing increased reliability and flexibility to the field operator, and at lower cost.